Revisiting the Selection of Stainless Steel in Water and Wastewater Treatment Environments: Part 2

Bacterial occurrence is observed in natural waters, from
both the surface and underground. Aerobic bacteria (Crenothrix, Gallionella)
primarily are encountered in the fresh surface waters, although anaerobic
sulfate-reducing bacteria and facultative strains also are encountered. Ground
waters are more likely to contain anaerobic and facultative bacteria, as well
as Gallionella than surface waters.3 Sulfate-reducing bacteria also are found
in seawater.3

Slime-producing bacteria are ubiquitous. They are typified
by the Pseudomonas genera. Members of this genera often are used to protect
farm crops from fungal growth and, as a result, are to be expected in
groundwater containing organics. However, these bacteria are highly adaptive.
Research several years ago indicated that these bacteria would grow in almost
any environment into which they were introduced. The Pseudomonas genera are
facultative anaerobes that can persist in oxygen-depleted environments by
breaking down complex hydrocarbons for the oxygen. In some circumstances,
Pseudomonas species will use nitrogen in the absence of oxygen.6 Some
Pseudomonas species are among the predominant denitrifiers, while others grow
prodigiously in and on tertiary treatment devices such as reverse osmosis and
electro-dialysis membranes and in sand or carbon filtration beds. Pseudomonas
species are capable of producing the polysaccharide matrix (biofilm) that acts
as a barrier, protecting the bacteria incorporated in the biofilm from harmful
substances such as disinfectants as well as trapping corrosive, metabolic
byproducts near the surface of the pipe. The fact that they are both
slime-formers and acid-producers, permits them to be the backbone of a biofilm
matrix.

One of the most significant Pseudomonads in drinking water,
and a predominant slime-former, is Pseudomonas aeruginosa (see Figure 3). This
is a waterborne genus capable of causing severe effects in compromised hosts.7
They can survive on minimal nutrients and tend to have a high negative surface
charge.9 It also is an acid former that liberates metal ions from the surface.8
Pseudomonas aeruginosa is known to cause pneumonia in humans, but also is
notorious for forming clumps—biofilms that resist chemical disinfectants,
antibiotics and the human immune system.8

Just how bad is this organism with respect to biofilm
formation? Vanhaecke, et al.9 conducted a study of permanent adhesion rates (a
precursor to biofilm formation) between an electro-polished surface and rougher
surfaces (400, 320 and 120 grit surfaces) at various pH values. While the
electro-polished steel adhesion rate was up to 100 times slower for a biofilm
than the 120 grit surfaces, in all cases permanent adhesion occurred in a
matter of minutes.9 Maximum adhesion rates occurred between pH 6 and 7.5, the
normal range for RO waters. The study concluded that adhesion of Pseudomonas
aeruginosa to stainless steel, even electro-polished stainless steel, occurred
in less than 30 seconds. Hydrophobicity was determined not to be a good
indicator for projecting biofilm adhesion to the steel.9

As previously noted, the slime incorporates other
detrimental bacteria. Filamentous iron-oxidizing bacteria often cited are
Sphaerotilus, Crenothrix and Leptothrix, while the stalked bacteria Gallionella
is found in tubercles as well. Iron bacteria, such as Gallionella species are
common in aerobic environments where iron and oxygen are present in the
groundwater and where ferrous minerals exist in the aquifer formation. These
bacteria attach themselves to the ferrous materials and create differentially
charged points on the surface, which in turn create corrosion problems. The
iron bacteria then metabolize the iron that is solublized in the process, often
creating ferric hydroxide to obtain the energy necessary for growth.5 Iron
bacteria tend to be rust colored or cause rust colored colonies on the pipe
surfaces.6 These colonies can grow on non-ferrous materials such as fiberglass
column pipes. (See Figure 4 for an example of a biofilm on blue fiberglass
pipe.)

Iron bacteria can cause damage to pipes due to the
deposition of iron compounds, resulting in clogging or tuberculation of pipes
and red water.10 Tubercles impede the penetration of biocides and corrosion
inhibitors, diminishing their effectiveness.5 Within tubercles, the anode is at
the bottom where access to oxygen is limited. Tubercle growth may be
accelerated by the presence of sulfate-reducing bacteria in the inner area of
the tubercle where conditions are anaerobic. Gallionella has been shown to be
dependent on a symbiotic relationship with sulfate-reducing bacteria in their
habitat for just this reason.5 The slime forming Pseudomonads also are
frequently found near tubercles.5 Electrolytic concentration cells are set up on
the metal surface beneath the tubercle as a result of poor adhesion and
irregular encrustations of organisms in the tubercles.

Anaerobic conditions will form under deposits, in crevices
and under the influence of BOD or COD independently of dissolved oxygen
content. Hydrogen sulfide production reduces pH and causes corrosion in
conjunction with chloride ions.3 The relevance of ennoblement of the corrosion
potential is to decrease pH and oxygen at the metal surface under the biofilm,
supplemented by porphyrin-type organo-heavy metal complexes or enzymes for the
respiratory system of the organisms forming the biofilm. Therefore, high
peroxide concentrations at the metal/biofilm interface will provide an
alternative cathodic reaction in addition to oxygen reduction. The effect of
ennoblement decreases the induction time for localized pitting and crevice
corrosion of susceptible alloys.5 The ennobling effect of the biofilm of the
corrosion potential would make the onset of pitting corrosion initiation more
probable.5

Sulfate-reducing bacteria often are responsible for the
hydrogen sulfide smell released when raw water is aerated. These bacteria are
common where sulfate exists naturally in the aquifer and will tend to form
black colonies on pipe surfaces. These bacteria are anaerobic and will live in
the oxygen-depleted environments under biofilms such as those produced by
Pseudomonas species and under tubercles. These bacteria produce acids as
metabolic byproducts that are corrosive to ferrous materials, and often are a
greater problem for low alloy stainless steel than for higher alloy stainless
steel.6 Sulfate-reducing bacteria metabolism brings to the metal/solution
interface several sulfur compounds of corrosive characteristics, either as a
metabolic product (sulfides, bisulfides or hydrogen sulfides) or intermediate
metabolic products (thiosulfates and polythionates) that are very corrosive to
carbon and steel.5 Since the bottom of the film is anaerobic, sulfate-reducing
bacteria can proliferate despite a measurable dissolved oxygen concentration in
the moving water.

The control of microbial populations in reverse osmosis
plant operations relies both on the control and management of disinfection as
well as good plant design, construction and mechanical material selection.11
The ability to observe and understand microbial concentrations and biofilm
growth concerns as they occur can eliminate more costly and problematic issues.
Pretreatment with chlorine is detrimental to the reverse osmosis process and
membranes. Where disinfection is practiced, bacterial growths are most often
controlled by chlorination—continuous chlorination of 0.2–0.3 ppm
normally is sufficient to deter bacterial growth unless silt or other debris in
the pipe are present.3 A 5 ppm chlorine residual commonly is used to minimize
growth potential in seawater sources. While chlorination is beneficial in
minimizing fouling and biogrowths, chlorination displaces the corrosion
potential in the ennoblement and, therefore, increases the crevice corrosion
potential.3

While bacteria populations in raw ground or surface water
supplies can be controlled to an extent by programs of routine disinfection
(and may mitigate gross concerns regarding biofouling), they cannot eliminate
the presence of organisms that are detrimental to ferrous materials.12 The use
of PVC or fiberglass casings can address these concerns, but the organisms will
continue to populate raw water intakes or wells. These inert materials can be
used as a pump column pipe, further reducing concerns, but these materials may
not be appropriate or practical in all situations.

Water Quality Testing Can Help

Obtaining representative samples of the feed water is the
most critical issue regarding the design of water treatment facilities and the selection
of materials. This requires sampling each feed water source. Water samples can
provide valuable information of the ionic composition and concentration of the
variety of microbial contaminants.5 Water quality parameters of importance in
selecting steel grades include

• Total
Dissolved Solids (TDS),

• Calcium
hardness,

• Magnesium
hardness,

• Total
hardness,

• Phenolphthalein
alkalinity,

• Methyl
orange alkalinity,

• pH,

• Chlorides,

• Sulfates,

• Silica,

• Iron,
and

• Dissolved
oxygen.3

These tests are recommended for selecting the steel to be
used in the treatment process. There also are samples for the treatment process
itself and the raw water supply appurtenances (such as wells). For example, the
City of Hollywood, Fla. has more than 25 wells in three separate wellfields
displaying two completely different water qualities. In addition, another
source is Broward County’s wellfield. Ideally water quality samples
should be taken from each well or at a minimum, a representative set of samples
from each different or suspected source should be collected. The water quality
analyses should include the following parameters and tests in addition to those
specifically required to predict corrosion potential as many of these analyses
also will provide information about corrosion potential.